1) Field of the Invention
The present invention relates to an ignition device for use in an internal combustion engine, which has an integral construction of a spark plug and an ignition coil.
2) Description of the Related Art
So far, as ignition devices for use in internal combustion engines which are constructed in a manner such that an ignition coil and a spark plug are integrated or united with each other, there have been proposed various types (see Japanese Patent Laid-Open No. 2000-252040 and European Patent Laid-Open No. 0907019). Moreover, Japanese Patent Laid-Open No. 2000-277232 discloses a construction in which a pressure detecting element, designed to detect a pressure of a combustion chamber, is incorporated into an ignition device for an internal combustion engine including an ignition coil and a spark plug which are in an integrated condition.
In general, in such types of ignition devices, a low-voltage side of a secondary winding is electrically connected through a connector terminal, an external wire harness and an internal combustion engine to an earth electrode of a spark plug in a manner such that the low-voltage side of the secondary winding is connected to the connector terminal and the wire harness connected to the connector terminal is connected to the internal combustion engine.
This requires one connector terminal and one wire harness for the connection of the low-voltage side of the secondary winding to the internal combustion engine.
In addition, in general, in such types of ignition devices, ignition coil components such as a primary winding, a secondary winding, a center core and an outer-circumferential core are accommodated in a resin-made case for the ignition coil, and spark plug components such as an insulator and a center electrode are accommodated in a metal-made case for the ignition plug.
However, because of the employment of the resin-made type as the case for accommodating the ignition coil components as mentioned above, this requires an outer-circumferential core which is for constituting a magnetic path for a magnetic flux generated by energizing the primary winding of the ignition coil.
The present invention has been developed with a view to eliminating these disadvantages of the ignition devices, and it is therefore an object of the invention to, in an ignition device for use in an internal combustion engine in which a spark plug and an ignition coil are integrated with each other and mounted in a cylinder head, reduce the number of parts to be put to use therein, that is, eliminating the need for the employment of a connector terminal and a wire harness which are to be used for the electrical connection of a low-voltage side of a secondary winding to the internal combustion engine and the need for the employment of an outer-circumferential core, thus fulfilling the diameter reduction requirement, cost reduction requirement or the like on the ignition device.
For this purpose, a first aspect of the present invention provides an ignition device for an internal combustion engine, comprising a spark plug (2) made to carry out an electric discharge between an center electrode (22) and an earth electrode (23) and an ignition coil (3) having a primary winding (31) and a secondary winding (32) for supplying a high voltage to the spark plug (2), with the spark plug (2) and the ignition coil (3) being mounted in a cylinder head of the internal combustion engine, the ignition device further comprising a tube-like case (1) having a section for accommodating the spark plug (2) and a section for accommodating the ignition coil (3), with the spark plug accommodating section and the ignition coil accommodating section being formed to be integrated with each other, and the case (1) being made of an electrical conductive material and one end portion of the secondary winding (32) and the earth electrode (23) being electrically connected to the case (1).
With this structure, a low-voltage side of the secondary winding and the earth electrode are electrically connected to each other through the case, which eliminates the need for a connector terminal and a wire harness to be used for connecting the low-voltage side of the secondary winding and the internal combustion engine. This enables the size reduction of the connector and enhances the reliability because of eliminating the need for the wriggling location of a wire harness for the electrical connection of the low-voltage side of the secondary winding to the internal combustion engine.
Moreover, this shortens the distance between the low-voltage side of the secondary winding and the earth electrode and reduces the number of connection places, thus reducing the resistance loss in the discharge circuit to enable efficient ignition.
In addition, in the ignition device according to the first aspect of the present invention, a pressure detecting element (4) is provided to detect the pressure in the combustion chamber of the internal combustion engine, and the pressure detecting element (4) is put in the case (1) and is electrically connected thereto and the case (1) is electrically connected to the cylinder head.
With this structure, one end portion (ground side) of the pressure detecting element is electrically connected through the case to the internal combustion engine, which eliminates the need for a connector terminal and a wire harness to be used for electrically connecting the one end portion (ground side) of the pressure detecting element to the internal combustion engine.
Furthermore, for achieving the above-mentioned purpose, a second aspect of the present invention provides an ignition device for an internal combustion engine comprising a spark plug (2) made to carry out an electric discharge in a combustion chamber of the internal combustion engine and an ignition coil (3) having a primary winding (31) and a secondary winding (32) for supplying a high voltage to the spark plug (2), with the spark plug (2) and the ignition coil (3) being mounted in a cylinder head of the internal combustion engine and a center electrode (22) being internally placed in a tube-like insulator (5), and further comprising a tube-like case (1) having a section for accommodating components of the spark plug (2) and a section for accommodating components of the ignition coil (3), with the spark plug component accommodating section and the ignition coil component accommodating section being formed to be integrated with each other, and the case (1) being made of a magnetic and electrical conductive metallic material.
With this structure, the case itself can function as an outer-circumferential core of the ignition coil, which eliminates the need for the separate employment of an outer-circumferential core unlike the conventional art, thus contributing greatly to the size reduction and cost reduction of the ignition device.
Moreover, since the case, including the ignition coil component accommodating section, is integrally formed through the use of the metallic material, the heat radiation property of the ignition coil components is further improvable, as compared with a type in which the ignition coil components are accommodated in a resin-made case.
In addition, in the ignition device according to the second aspect of the present invention, the ignition coil (3) includes a bar-like center core (33) and a slit (15) extending in an axial direction of the center core (33) is made in a section surrounding the center core (33) in the case (1).
This avoids a loss stemming from a ring current generated by a variation of magnetic flux.
Still additionally, in the ignition device according to the second aspect of the present invention, the case (1) is electrically connected to the cylinder head.
In this case, since the ignition coil section is covered with the metal-made case connected to the cylinder head serving as the ground, ignition noises developing in the ignition coil become hard to leak to the external owing to this shielding function of the case.
The reference numerals in parentheses for the above-mentioned means respectively indicate the corresponding relations to the concrete means in embodiments which will be described later.